Normal and Binomial. Distributions


 Ernest Ferguson
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1 Normal and Binomial Distributions Library, Teaching and Learning 14
2 By now, you know about averages means in particular and are familiar with words like data, standard deviation, variance, probability, sample, population You must know how to use your calculator to enter data, and from this, access the mean, standard deviation and variance You need to become familiar with the various symbols used and their meanings be able to speak the language: Sample statistics are estimates of population parameters: symbol used for the symbol used for the population parameter sample statistic mean x standard deviation s variance s standard error n You should appreciate that the analysis and interpretation of data is the basis of decision making. For example o Should the company put more money into advertising? o Should more fertilizer or water be applied to the crop? o Is it better to use Brand A or Brand B? etc There are many analytical processes. Which process you use depends on o What type of data you have discrete or continuous o How many variables  one, two, many o What you want to know Tests and Examination preparation ractise on a regular basis set aside, say, half an hour each night or every second night, and/or 3 times during the weekend rather than a whole day or several hours just before a test. Make sure your formula sheet is with you as you work, so that you become familiar with the information that is on it. The following sections show summaries and examples of problems from the Normal distribution and the Binomial distribution. Best practice For each, study the overall explanation, learn the parameters and statistics used both the words and the symbols, be able to use the formulae and follow the process. s n
3 Normal Distribution Applied to single variable continuous data that is distributed approximately normally. e.g. heights of plants, weights of lambs, lengths of time f (frequency) This distribution is graphed like: Used to calculate the probability of occurrences less than, more than, between given values e.g. the probability that a randomly selected plant will be less than 7mm the probability that a randomly selected lamb will be heavier than 7kg the probability that the time taken will be between 1 and 1 minutes Standard Normal tables give probabilities  you will need to be familiar with the Normal table and know how to use it. First need to calculate how many standard deviations above (or below) the mean a particular value is, i.e., calculate the value of the standard score or Zscore. Use the following formula to convert a raw data value, X, to a standard score, Z : Z X Xvalue eg. Suppose a particular population has m= 4 and σ =. Find the probability of a randomly selected value being greater than The Z score corresponding to X = 6 is Z 1. (Z=1 means that the value X = 6 is 1 standard deviation above the mean.) Now use standard normal tables to find (Z>1) =.6587 (more about this later). rocess: o Draw a diagram and label with given values i.e. population mean, pop s.d. and X ( raw ), score o Shade area required as per question o Convert raw score X to standard score o Use tables to find probability, eg Z z p. o Adjust this result to required probability Z using formula That is, find p (an event) given a population mean and s.d. and a raw score. 3
4 Example 1 Wool fibre breaking strengths are normally distributed with mean m = 3.56 Newtons and standard deviation, σ = What proportion of fibres would have a breaking strength of or less? Draw a diagram, label and shade area required: X=14.45 xm = 3.56 sσ = 4.55 Convert raw score X to standard score Z : Z That is, the raw score of is equivalent to a standard score of .. It is negative because it is on the left side of the curve. Use tables to find probability and adjust this result to required probability: X Z.. 8 In this case there is no adjustment needed, since the shading in the diagram is similar to that shown in the standard normal table. That is the proportion of fibres with a breaking strength of or less is.8%. Note: Standard normal tables come in various forms. The ones used for these exercises show the probability of Z being less than z, i.e., (Z<z). Some forms of standard normal tables give (<Z<z). Make sure you can use your table appropriately. Examples: Use the following examples to check your understanding of how to use the tables and make adjustments: Example A Example B Example C (Z>1.68) = 1 (Z<1.68) (Z<.44) =.997 (1.77<Z<) =.5 (Z<1.77) = = =.465 = Example D Example E Example F (Z>1.68) = 1 (Z< 1.68) (Z<.3) =.1 (1.1<Z<.97) =(Z<.97)(Z<1.1) = = =.9535 =
5 Example G (.11<Z<1.88) = (Z<1.88)  (Z<.11) = =.955 Example A machine produces ball bearings, the weights of which are normally distributed with a mean of 5 mg and a standard deviation of 8 mg. (a) Find the percentage of ball bearings produced which weigh more than 49 mg. (b) If 675 ball bearings are chosen at random, how many could be expected to weigh more than 49 mg? Solution: (a) Let X be the weight of a ball bearing. X 49 = Z Z 1.5 Inverse process: (to find a value for X, corresponding to a given probability) o Draw a diagram and label o Shade area given as per question o Use probability tables to find Z score o Convert standard score Z to raw score [Change raw score to standard score] = 1  (Z<1.5) = =.8944 ie 89.44% (b) 89.44% of 675 = 63.7 or 64 X using inverse formula Example: Carrots entering a processing factory have an average length of 15.3 cm, and standard deviation of 5.4 cm. If the lengths are approximately normally distributed, what is the maximum length of the lowest 5% of the load? (I.e., what value cuts off the lowest 5 %?) Draw a diagram, label and shade area given as in question: =.5 x 15.34mm m = 15.3cm σ s= cm mm Use standard Normal tables to find the Z score corresponding to this area of probability. Convert the standard score Z to a raw score X using the inverse formula: X Z 5
6 For Z z.5, the Normal tables give the corresponding zscore = (Negative because it is below the mean.) Hence the raw score is: X Z i.e., the lowest maximum length is 6.4cm Exercises 1. For a particular type of wool the number of crimps per 1cm follows a normal distribution with mean 15.1 and standard deviation (a) What proportion of wool would have a crimp per 1 cm measurement of 6 or less? (b) If more than 7% of the wool has a crimp per 1 cm measurement of 6 or less, then the wool is unsatisfactory for a particular processing. Is the wool satisfactory for this processing?. The finish times for marathon runners during a race are normally distributed with a mean of 195 minutes and a standard deviation of 5 minutes. a) What is the probability that a runner will complete the marathon within 3 hours? b) Calculate to the nearest minute, the time by which the first 8% runners have completed the marathon. c) What proportion of the runners will complete the marathon between 3 hours and 4 hours? 3. The download time of a resource web page is normally distributed with a mean of 6.5 seconds and a standard deviation of.3 seconds. a) What proportion of page downloads take less than 5 seconds? b) What is the probability that the download time will be between 4 and 1 seconds? c) How many seconds will it take for 35% of the downloads to be completed? 4. otassium blood levels in healthy humans are normally distributed with a mean of 17. mg/1 ml, and standard deviation of 1. mg/1 ml. Elevated levels of potassium indicate an electrolyte balance problem, such as may be caused by Addison s disease. However, a test for potassium level should not cause too many false positives. What level of potassium should we use so that only.5 % of healthy individuals are classified as abnormally high? 5. Household consumption of water in a particular suburb is found to be approximately normal with a mean of 79 litres per day and a standard deviation of 9 litres per day. a) What is the probability that a randomly selected household will use more than 36 litres per day? b) What is the probability that a randomly selected household will use between and 35 litres per day? c) The council supplying the water want to charge the top 1% of water users a different rate from the rest. What is the minimum consumption (litres per day) that would put a household into the top 1%? 6
7 Worked solutions (Normal Distribution) Z 4.79 X 6 x m = 15.1, 15.1 sσ = (X < 6) = (Z < 1.9) =.87 (a) That is the proportion of wool with a crimp of 6 or less is.87%. (b) Yes, this is satisfactory since.87% is less than the stated 7% of the wool.. (a) (b) X=18 mean m = = s.d. = 5 σ = x 18 z.6 5 Z Z 1.41 X 195 Hence X or 16 mins i.e. probability of a runner taking less than 3 hours (18 mins) is.743 (c) X=18 X= 4 mean = 195 s.d. = X 18 Z.6 5 Z i.e. proportion of runners taking between 3 and 4 hours (18 and 4 minutes) is approximately 7%. Z X 4 Z Z
8 3. X Z.65.3 X = 5 sec x = 6.5 sec m = 6.5 sec s =.3 sec σ =.3 sec Z ( X 5) (a) That is, approximately 6% of the downloads take less than 5 seconds. (b) Z 1.9 X = 4 sec.7978 x Z 1.5 m = sec sec X = 1 sec σ s = =.3.3 sec sec Z X 4 Z X 1 Z That is, about 8% of downloads take between 4 and 1 seconds. (c) p =.35 (Inverse problem) For.35, Z.38 X x m = 6.5 sec sec σ s =.3 sec That is, 35% of downloads are completed in 5.6 seconds or less. 4. (This is an inverse problem) 17, 1 =.5 The z value (standard normal score) that cuts off the upper.5% is 1.96, so we find the value for potassium which is 1.96 SDs above the mean for a healthy person. x x 17 Start with Z ie 1.96 x That is X = mg/1 ml. x 17, s 1 m = 17, σ = 1 8
9 5. (a) (b) X= 36 mean = 79 s.d. = 9 x= x=35 mean = 79 s.d. = 9 X 36 Z Z X Z 9 Z Z.79 X 35 Z Hence Z (c) mean=79 s=9 Z =.1 p.1 Z 1.8 X 79 Hence, X ie 1% of households use over 394 litres. 9
10 Binomial Distribution Applied to single variable discrete data where results are the numbers of successful outcomes in a given scenario. e.g.: no. of times the lights are red in sets of traffic lights, no. of students with green eyes in a class of 4, no. of plants with diseased leaves from a sample of 5 plants Used to calculate the probability of occurrences exactly, less than, more than, between given values e.g. the probability that the number of red lights will be exactly 5 probability that the number of green eyed students will be less than 7 probability that the no. of diseased plants will be more than 1 arameters, statistics and symbols involved are: population sample statistic symbol parameter symbol probability of success p sample size N n Other symbols: X, the number of successful outcomes wanted n C, x or n C sample of size n. The formula. r : the number of ways in which x successes can be chosen from a n C key on your calculator can be used directly in the r Formula used: No. of successes Combination of x successes from n trials number of failures n x nx X x C. p.(1 p) x random variable X probability of success probability of failure Read as the probability of getting x successes is equal to the number of ways of choosing x successes from n trials times the probability of success to the power of the number of successes required times the probability of failure to the power of the number of resulting failures. 1
11 Example An automatic camera records the number of cars running a red light at an intersection (that is, the cars were going through when the red light was against the car). Analysis of the data shows that on average 15% of light changes record a car running a red light. Assume that the data has a binomial distribution. What is the probability that in light changes there will be exactly three (3) cars running a red light? Write out the key statistics from the information given: p.15, n, X 3 Apply the formula, substituting these values: 3 17 X 3 C That is, the probability that in light changes there will be exactly three (3) cars running a red light is.4 (4%). ractice (Binomial Distribution) 1 Executives in the New Zealand Forestry Industry claim that only 5% of all old sawmills sites contain soil residuals of dioxin (an additive previously used for antisapstain treatment in wood) higher than the recommended level. If Environment Canterbury randomly selects old saw mill sites for inspection, assuming that the executive claim is correct: a) Calculate the probability that less than 1 site exceeds the recommended level of dioxin. b) Calculate the probability that less than or equal to 1 site exceed the recommended level of dioxin. c) Calculate the probability that at most (i.e., maximum of) sites exceed the recommended level of dioxin. Inland Revenue audits 5% of all companies every year. The companies selected for auditing in any one year are independent of the previous year s selection. a) What is the probability that the company Ross Waste Disposal will be selected for auditing exactly twice in the next 5 years? b) What is the probability that the company will be audited exactly twice in the next years? c) What is the exact probability that this company will be audited at least once in the next 4 years? 3 The probability that a driver must stop at any one traffic light coming to Lincoln University is.. There are 15 sets of traffic lights on the journey. a) What is the probability that a student must stop at exactly of the 15 sets of traffic lights? b) What is the probability that a student will be stopped at 1 or more of the 15 sets of traffic lights? 3 11
12 Worked solutions (Binomial Distribution) 1. (a) n, p.5, X 1 X X C (b) (c) n, p.5, X n, p.5, X X X X X X 1 X C X, X,1,1, C X,1, (a) p.5, n 5, X 5 3 X C (b) in two years C (c) i.e. % i.e..5% at least once X X 1 C (a) (b) p., n 15, X X 1 1 X 1 1 X X C C i.e. 3% i.e. 97% 1
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